1. Field of the Invention
The invention relates to an electrophotographic image forming apparatus using an electrophotographic printing method, and particularly to an electrophotographic image forming apparatus such as a copying machine, a printer or a facsimile apparatus.
2. Description of the Related Art
An electrophotographic photosensitive member (hereinafter suitably simply referred to as the “photosensitive member”) is required to be provided with sensitivity, an electrical characteristic and further, an optical characteristic conforming to an electrophotographic process applied thereto. Particularly, in the photosensitive member for repetitive use, electrical and mechanical extraneous forces such as charging, exposure, development, transfer and cleaning are directly applied to the surface of the photosensitive member and therefore, durability to these extraneous forces is needed.
In an image forming apparatus of an electrophotographic type or an electrostatic recording type (such as, for example, a copying machine, a printer or a facsimile apparatus), a corona charger or a roller charger is often used as a charging device for uniformly charging (including a charge eliminating process) an image bearing member (a member to be charged) such as an electrophotographic photosensitive member or an electrostatic recording dielectric member to a predetermined polarity and predetermined potential.
As the electrophotographic photosensitive member, a photosensitive member using an organic material is popular from such advantages as a low price and productivity. The main current of the organic photosensitive member (OPC photosensitive member) is a photosensitive member of a function separate type comprising a charge generating layer containing an organic photoconductive dye and a pigment and a charge transport layer containing a photoconductive polymer and a low molecular organic photoconductive substance, the charge generating layer and the charge transport layer being laminated. Most of its surface layer comprises a molecule dispersed polymer having an organic photoconductive substance dispersed in a polymer, and its mechanical strength depends on the polymer, and with the recent high quality of image and longer life, its durability could not be said to be sufficient.
In contrast with this, in achieving the higher durability of the photosensitive member, it is known that the use of hardenable resin for the surface layer is effective (see Japanese Patent Application Laid-open No. H02-127652, Japanese Patent Application Laid-open No. H05-216249, and Japanese Patent Application Laid-open No. H07-72640). When hardenable resin is used for the surface layer, as compared with thermoplastic resin or the like, the mechanical strength increases and the surface layer becomes difficult to scrape off and also becomes difficult to injure, and thus the life of the photosensitive member becomes longer.
It is known that when hardenable resin is used for the surface layer of the photosensitive member, it is useful to use an electron ray as hardening means therefor, from the viewpoint of durability to the injury and scraping-off of the surface layer (see Japanese Patent Application Laid-open No. 2000-66425).
Therefore, use is made of a photosensitive member having a surface layer hardened by an electron ray and an image forming apparatus in which the photosensitive member is charged by a corona charging method is constructed, whereby there can be established an electrophotographic system which can greatly extend the life of the photosensitive member against the injury and scraping-off.
On the other hand, in an image forming apparatus, it is known that it is useful for the obtainment of a higher quality of image to make the film thickness of the charge transport layer, or the protective layer and the charge transport layer of the photosensitive member small (see Japanese Patent Application Laid-open No. H08-272197).
Accordingly, to achieve higher durability and a higher quality of image in the image forming apparatus, it becomes necessary to provide a photosensitive member having a photosensitive member surface layer hardened by an electron ray and in which the film thickness of a charge transport layer or a protective layer and the charge transport layer is small.
However, in a case where use is made of a photosensitive member in which the film thickness of a surface layer (a protective layer or a charge transport layer is small, when continuous sheet supply endurance was effected under a low humidity environment, there arose the problem that image density lowered as the number of sheets was increased. Also, this phenomenon has dependency on the drum surface temperature under a low humidity environment, and exhibited the tendency of becoming more remarkable when the temperature was low.
Also, when hardenable resin is used for the surface layer of the photosensitive member, the mechanical strength thereof increases and the surface layer becomes difficult to scrape off and the life of the photosensitive member becomes longer, while on the other hand, the reformability of the surface of the photosensitive member lacks, and image deletion resulting from a discharge product or the like adhering to the surface layer having absorbed humidity becomes liable to occur.
The problem of image deletion has heretofore been prevented from arising by maintaining the surface temperature of the photosensitive member at a high temperature under a high humidity environment. Accordingly, if even under an environment of low humidity in which a density fluctuation occurs, the surface temperature of the photosensitive member is likewise maintained at a high temperature, the both problems of density fluctuation and image deletion will be solved.
However, in an image forming apparatus particularly having a construction in which an intermediate transfer belt (rotary transfer member) which is a belt-shaped intermediate transfer member contacts with a photosensitive member at a primary transferring position, or a construction in which a predetermined amount of wind is blown against the photosensitive member during sheet supply to effectively remove a discharge product in a primary charger, the heat of the surface of the photosensitive member is gradually taken away from the timing at which sheet supply has been started, and the temperature of the photosensitive member falls.
There will arise no problem if the environment of use is under a high temperature, but the fall of the temperature of the photosensitive member is particularly vehement in an environment of an ordinary temperature of the order of 20-25° C. Thus, by setting the surface temperature of the photosensitive member at the initial stage of sheet supply at a high temperature, it is possible to maintain the temperature of the photosensitive member at a temperature whereat a density fluctuation is little and image deletion does not occur. However, when the fall of the temperature of the photosensitive member is great, the amount of charge generation of the photosensitive member fluctuates, and this becomes the factor of the fluctuation of density.
A heater for adjusting the drum temperature effects the simple control of charging over so as to become OFF if the value detected by a drum temperature sensor is high relative to a set target temperature, and to become ON if the aforementioned value is low relative to the set target temperature. However, even if the drum surface temperature is judged to be low and the heater is in its operative state, the amount of electric power during sheet supply is limited, and when it is limited to a low level, it becomes impossible to maintain the target temperature.